Efficacy vs. Effectiveness
First things first. Turns out “efficacy” and “effectiveness” don’t mean the same thing, even though the words are often thrown around interchangeably, even by reliable sources! The distinction between these two words is v. important!
Birth control efficacy refers to how well each form of birth control fulfills its intended duty (preventing pregnancy) in perfect-use situations. A perfect-use situation is not a real life situation, but instead something like a clinical trial in which only the months or cycles where the birth control method was used consistently and correctly are taken into the equation. It accounts for pregnancies that occur even with this consistent and correct use of the prescribed method.
Effectiveness on the other hand, refers to how well each form of birth control prevents pregnancy in typical-use situations, meaning, the average Joe at home in a real life, day-to-day setting. This takes into account human error and covers all pregnancies and months, keeping in mind the possibility of human inaccuracies or imperfect use. This includes scenarios like delaying the exchange of your implant and not taking your pill at the same time consistently or skipping a few days here and there.
Effectiveness and Efficacy are measured in a couple of ways:
THE PEARL INDEX
This is the most widely used method.
There are two different equations that can be used, both considered equally valid and both based on the assumption of an “average” cycle (28 days).
Pearl Index Equation No. 1:
The 12 here is in reference to there being 12 months in a year.
Pearl Index Equation No. 2
The 13 accounts for the number of cycles in a year, instead of months. The equation assumes a 28 day menstrual cycle.
If a method has a low Pearl Index, the likelihood of a pregnancy is also low and so the effectiveness or efficacy of the method is high.
The Life Table aka a Decrement Table measures efficacy and effectiveness over each month of use AND a whole year of use (2). This measurement can uniquely provide cumulative failure rates for a specific length of exposure to the method of birth control. Meaning, you can look at periods of time other than 12 months or 13 cycles. The Pearl Index can’t provide this.
Pearl Index vs Life Tables
While the Pearl Index is universally accepted and the formula is easy to use, it has more than a few downsides.
First, it assumes a constant failure rate over time; meaning that, for example, the rate at which failure occurs is the same in the first month of use as it is in the ninth month of use. This is tricky to assume because numerous things can alter the failure rate over time. Fertility is one such factor. For example, couples that are most fertile are more likely to become pregnant sooner in the study, leaving the least fertile couples by the study’s end (3). Therefore, it is difficult to say whether the birth control was working, or if the person’s fertility was simply lower, making them less likely to become pregnant regardless.
Moreover, as a study progresses, people become more comfortable and apt at using their birth control method (3). So, the rate of pregnancies may become lower as time moves on simply because they have gotten better at the method, also altering the failure rate over time.
Then there are issues with the equation itself. The longer the study length, the lower the Pearl Index will be – and therefore, comparisons of Pearl Indexes from studies of different lengths cannot be accurate. It also assumes that every person in the study has a certain cycle-length. One study estimates that only 13% of menstruators actually have that “average” 28 day menstrual cycle. (4)
Finally, only the pregnancies reported by the women* themselves have traditionally been included in this computation, and some may have gotten pregnant and failed to report it to the study. (5)
Many would argue that Life Tables are the more accurate method for measuring birth control efficacy because they take into account time-related biases such as fertility and skill in administering said method. In fact, study after study points out The Pearl Index’s flaws (6), and even shows differences in Pearl Index results from study to study (7). Yet, the Pearl Index calculations are still used by most major institutions and pharmaceutical companies to talk about birth control efficacy and effectiveness.
So Is B.C as effective as advertised?
It’s easy to take a company or institution’s word at face value regarding a birth control method’s efficacy. But, it is important to remember that “efficacy” and “effectiveness” aren’t the same, and they are often used, even by “trusted” sources, incorrectly or interchangeably.
For example, take the Depo-Provera shot. If you go to WebMD.com. They state that “The Depo-Provera shot is 99% effective in preventing pregnancy”. While it is true that the efficacy rate of the shot is 99%, it’s effectiveness (typical use rate) is actually 94%. (8) This is not stated anywhere on WebMD’s page. What’s more, if you go to the website of the pharmaceutical company that makes Depo Provera they too only talk about results from clinical trials, not typical use. #helpful
In addition to understanding whether you are looking at typical-usage rates or clinical trial rates, it is also important to understand the shortcomings of clinical research overall. Calculations are made based on the observations within a given sample population. Thus studies of different populations using the same contraceptive will yield different values for the index. The culture and demographics of the population being studied, and the instruction technique used to teach the method, can have significant effects on its failure rate (9,10). Historically, trial populations don’t represent the wider population.
Ummm so should I not pay attention to birth control efficacy numbers at all?
No!! Understanding your birth control is SO important. This is just a reminder that knowing your own body, lifestyle, and sexual activities is a very, very vital part of the equation as well. Because for ALL birth control methods what is certainly true is that the more perfectly you use it, the more effective it is. So, while it may be confusing whether the method has a 99% or 94% effectiveness rate, what you can reliably think about is which method you, as an individual, can actually use most effectively, correctly, and reliably based on your unique lifestyle and body.
General rules of thumb?
- Methods that require less user participation (for example, the IUD) have lower rates of human failure and therefore higher effectiveness rates across the board. Methods like taking a daily pill and using the Fertility Awareness Method fall more on the side of “heavy user participation” and therefore effectiveness tends to be lower.
- It is always important to talk to your practitioner about potential risks and side effects in addition to effectiveness. Those are just as important a part of the discussion!
Written by: Clare McCammon
Reviewed by: Rose Stevens, PhD Candidate researching reproductive hormone variation and contraceptive side-effects.
All content found on this Website, including: text, images, audio, or other formats, was created for informational purposes only. The Content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.
1.“How Effective Is Your Contraception?” New Zealand Family Planning , April 1, 2014. https://www.familyplanning.org.nz/news/2014/how-effective-is-your-contraception.
2. Schreiber, Courtney A, and Kurt Barnhart. “Chapter 36: Contraception.” In Yen and Jaffe’s Reproductive Endocrinology, 7th ed., 890, 2014.
3. Trussell, James, and David Portman. “The Creeping Pearl: Why Has the Rate of Contraceptive Failure Increased in Clinical Trials of Combined Hormonal Contraceptive Pills?” Contraception 88, no. 5 (2013): 604–10. https://doi.org/10.1016/j.contraception.2013.04.001
4. Bull, Jonathan R., Simon P. Rowland, Elina Berglund Scherwitzl, Raoul Scherwitzl, Kristina Gemzell Danielsson, and Joyce Harper. “Real-World Menstrual Cycle Characteristics of More than 600,000 Menstrual Cycles.” Nature News. Nature Publishing Group, August 27, 2019. https://www.nature.com/articles/s41746-019-0152-7
5. Rafie, S. (2020). Updates in Contraception. [ebook] Available at: https://www.accp.com/docs/bookstore/acsap/a15b2_m1sample.pdf. Accessed 27 Feb. 2020
6. Trussell, James, and David Portman. “The Creeping Pearl: Why Has the Rate of Contraceptive Failure Increased in Clinical Trials of Combined Hormonal Contraceptive Pills?” Contraception. U.S. National Library of Medicine, November 2013. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3795840/.
7. Lobo Abascal, Paloma, Vesna Luzar-Stiffler, Silvana Giljanovic, Brandon Howard, Herman Weiss, and James Trussell. “Differences in Reporting Pearl Indices in the United States and Europe: Focus on a 91-Day Extended-Regimen Combined Oral Contraceptive with Low-Dose Ethinyl Estradiol Supplementation.” The European journal of contraception & reproductive health care : the official journal of the European Society of Contraception. U.S. National Library of Medicine, 2016. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5533102/.
8. Parenthood, Planned. “Depo-Provera: Birth Control Shot: Birth Control Injection.” Planned Parenthood. Accessed February 27, 2020. https://www.plannedparenthood.org/learn/birth-control/birth-control-shot.
9. Trussell J (1991). “Methodological pitfalls in the analysis of contraceptive failure”. Statistics in Medicine. 10 (2): 201–220. doi:10.1002/sim.4780100206. PMID 2052800.